1. Field of the Invention
The present invention relates generally to integrated circuits, and more specifically to a structure, and method for forming same, useful in the fabrication of high resistance regions and low resistance regions in a single polycrystalline silicon layer.
2. Description of the Prior Art
CMOS SRAMs often use a four transistor cell design having resistive load devices. This design is used in order to save chip layout area over the traditional six transistor cell design. Two N-channel transistors are used to form a cross-coupled latch, while two additional N-channel transistors are used to provide access to the cell for reading and writing data. Two load devices are connected between the N-channel transistors in the latch and the power supply.
In the prior art, the resistive load devices are formed after formation of the transistors. After the transistors have been formed, a dielectric layer is deposited and contact openings are formed to the substrate. A second polycrystalline silicon layer is deposited and light doped N-type to achieve a resistivity in the range of 10.sup.6 to 10.sup.13 ohms/square. This blanket implant determines the load resistor value.
The second polycrystalline silicon layer also serves to provide interconnect between various portions of the integrated circuit. It can be used for the V.sub.cc supply connected to the load resistors. It may also be used for local interconnect between various portions of the device.
The interconnect portions of the second polycrystalline silicon layer must have a relatively low resistivity. In order to lower the resistance of the interconnect regions, the locations used for the resistive load devices are masked and a highly doped N-type implant is made. This implant lowers the resistivity of the interconnect regions to, typically, a range of between 100 and 1000 ohms/square. The second polycrystalline layer is typically patterned to define the resistive devices and interconnect signal lines after all of the implants have been made.
It is desirable to use a single polycrystalline silicon layer for both the resistive load devices and interconnect lines. Such an approach is more economical than forming such regions from separate polycrystalline silicon layers, and also results in a relatively smoother chip surface. However, this approach has an important drawback in that the resistivity of the resistive element regions and the interconnect regions are somewhat related. It has heretofore been difficult to form both very high resistance and very low resistance regions in a single polycrystalline silicon layer.
It would be desirable to provide a structure and method for fabricating very high resistance and very low resistance regions in a single polycrystalline silicon layer. It would be further desirable for a technique to form such regions to be compatible with current technology, and add a minimal amount of complexity to device process flows.